Download femtosecond laser ablation of dielectrics

Mechanisms of Femtosecond Laser Ablation of Dielectrics Revealed
by Double Pump - Probe Experiment.
Stéphane Guizard1, Nikita Fedorov1, Alexandros Mouskeftraras1, Sergey Klimentov2,
1- Laboratoire des Solides Irradiés, CEA-IRAMIS, CNRS, Ecole Polytechnique,
91128 Palaiseau, France,
2- General Physics Institute of the Russian Academy of Sciences, Vavilova St 38,
11991 Moscow,
Russia.
e-mail: [email protected]
Femtosecond lasers have witnessed an impressive progress during the past few decades, leading to a
growing number of industrial applications nowadays. Their success, among many other aspects, is also
due to their unique capacity to precisely micro-machine any type of materials, and in particular
transparent dielectric materials.
Despite the fact that the basic mechanisms of the processes have been widely studied, important
questions still remain to be conclusively answered. The difficulty arises from the large number of
phenomena involved during and after the interaction: non linear excitation by multiphoton absorption
or tunnelling ionisation, further excitation of free carriers by the laser field, followed by a variety of
relaxation processes: electron phonon collisions, plasma expansion, shock wave and heat propagation,
etc.
Time resolved experiment is the only method capable of observing the different steps of the complex,
dynamic interaction. However, because the modification of the solid takes place during the pulse,
leading to an intricate evolution of the laser field and the optical properties of the solid, pump-probe
experiments could not reveal unambiguously the different mechanisms involved in the interaction [1,
2, 3].
To overcome all these obstacles, we have performed experiment in which the samples (SiO 2 and
Al2O3) are excited by a pair of laser pulses. The first pulse, with fixed duration of 50 fs, is the second
harmonic of a Ti-Sa laser, and excites a well controlled density of carriers in the conduction band.
Then we send a second pulse, at 800nm. The energy and pulse duration of this second pulse can be
modified in a wide range, as well as the time delay relative to the first pump pulse. The aim of this
second pulse is to re-excite the carriers previously promoted in the conduction band. Then the
evolution of the solid is monitored, under conditions where breakdown is achieved and below, with
the time-resolved interferometry technique [4] which allows to measure the density of carriers in the
solid, and to follow their relaxation. Thanks to its great flexibility, this double pulse technique allowed
us to observe and study all the different mechanisms that can play a role in the ablation and machining
processes: multiphoton excitation, heating of free carriers, multiplication of carriers due to impact
ionisation- i.e. the avalanche mechanism which is very often invoked but has never been directly
observed, de-trapping of self-trapped exciton- in the case of SiO2.
Our results show that different behaviours can be observed in Al 2O3 and SiO2 for instance, meaning
that the breakdown and ablation processes of wide band gap materials cannot be simply explained by a
model taking into account the laser induced plasma. The relative importance of all the different basic
processes that we could reveal, and the consequence of these experimental results on predictive
models, will be discussed in details.
[1] Quéré F, Guizard S, Martin P, Europhys. Lett. 56, p. 138-144, 2001.
[2] V.V. Temnov, et al., Phys. Rev. Lett. 97, p. 237403, 2006.
[3] D. Grojo, M. Gertsvolf, S. Lei,et al, Phys Rev. B, 81, 212301, 2010.
[4] Audebert P., Daguzan Ph., Guizard S., et al. Physical Review Letters 52, p. 1994.